This September, take a quick break from hunting on the ground for mushrooms and glance up at the branches of American beech trees (Fagus grandifolia) to find the fungus of the month, Scorias spongiosa. It will be growing in several areas in the eastern United States and Canada. Scorias spongiosa is one of the Capnodiales sooty molds, a group of ascomycetes that grow on insect honeydews or sugary plant exudates (Hughes 1976, Reynolds 1978, Schoch et al. 2006). The sooty molds generally grow in thin black layers on leaves on which aphids have dripped their honeydew exudate-- essentially their poop. There are many species of trees that can show sooty molds on their leaves, including many hardwoods, such as beech, magnolia, maple, oak, elm, basswood, willow, and walnut. Conifers such as spruces, pines and firs may also be affected.

Sooty molds do not actually penetrate the leaves, so they are not parasitic on the plant. However, they can harm some plants by blocking photosynthesis, slowing growth, and hurting fruit production. However, sooty molds may be beneficial in some ways. Recently, Jouraeva et al. (2006) found that leaves covered in sooty molds adsorb more polycyclic aromatic hydrocarbons and heavy metals from air than clean leaves. Thus they may be more efficient at cleaning up polluted air. Unlike many other species of sooty molds, Scorias spongiosa is restricted to one species, American beech, and can grow to a much larger size. Some specimens on tree trunks can grow to be the size of footballs. To the left you can see some average sized specimens, each about 15 cm long. The top specimens is younger, still in its yellow stage, and the lower specimen is much more mature.

Most of the sooty molds are generalists and can be found on the honeydews of several insects, but Scorias spongiosa appears to be a specialist. It grows under colonies of the beech blight aphid, Grylloprociphilus imbricator, also sometimes called the beech woolly aphid. The reason that Scorias spongiosa is so much larger than its relatives is because of the colonial habit of the beech aphids compared with the more or less solitary habit of most other aphids. As shown in the video, the beech aphid colonies are concentrated in only a few places, so their honeydew falls on just a few branches and leaves, leaving most of the tree free of the mold. Because of this relatively larger concentration of the honeydew, the S. spongiosa mycelium can grow to be much larger than just a thin film on leaves. Like the other sooty molds, Scorias spongiosa does not seem to harm the beech trees, though effects on its host have not been fully studied. You can learn more about sooty molds, including some prevention and remediation measures here.

Early in September, it is easiest to find Scorias spongiosa by first locating the beech aphids and then search for the sooty mold beneath them. In North Carolina, for example, colonies of the beech blight aphid will be on lower branches early in the month and may last well into October. Sometimes you can even find the colonies aggregating as early as July. Look for branches of a beech tree that appear to be covered in cotton. A closer examination will show thousands of small aphids each with a tuft of wax at the end of their abdomens. Scorias spongiosa will start growing in a thin layer of yellow-brown tufts of asexual, flask-shaped, spore-bearing structures called pycnidia (Reynolds 1978) where the beech aphid honeydew falls. As more and more honeydew accumulates, the fungus will grow larger until it looks like a large yellow sponge sitting on the beech branches or leaves. In this stage, it produces asexual conidia in liquid droplets from its pycnidia. In this stage it is very spongy, kind of the texture of a gelatinous gummi bear. Later, it will blacken and produce sexual spores in pseudothecial ascocarps. A pseudothecium is a flask-shaped structure that contains ascospores. These pseudothecia are embedded in sterile tissue called a stroma, which in this case is the spongy part of the fungus . (another fungus that has pseudothecia is Venturia inaequalis, the apple scab fungus. At this stage the fruiting structure typically becomes thicker and harder. The fungus is very durable, so if you find it in the yellow stage on a tree, you can return each week to watch it develop. In the Duke Forest in Durham, North Carolina, sometimes it stays on some trees all winter and into spring, but it's usually in the black stage by then.

We hope you enjoyed learning about Scorias spongiosa, a cool and weird fungus with a very interesting life history. If you have American beech trees near you, you probably can find this fungus and its aphids. There are a lot of ecological and taxonomic problems to be worked out, and that's what Hannah is working on. This month's co-author is Hannah Reynolds, a graduate student at Duke University. She writes, "I am studying the ecology and biogeography of this sooty mold and the beech blight aphid and will be collecting specimens across the country this September. If you see it in your area, please let me know by emailing me at: . I would also greatly appreciate any specimens you could send me. The fungi should be dried and the aphids stored in ethanol. Please include your name and the location you collected (and GPS coordinates if you have them)."